Hello, world

K&R Exercise 1-1: run the classic hello, world program on your machine, then deliberately leave pieces out and read what the compiler says.

Write the program

Create a file called hello.c:

#include <stdio.h>

int main(void)
{
    printf("hello, world\n");
    return 0;
}

That is the smallest honest C program that talks to the outside world: include the standard I/O header, enter at main, print a line, leave cleanly.

Compile and run

In Terminal, from the folder that holds hello.c:

gcc hello.c -o hello
./hello

You should see:

hello, world

If gcc is missing, finish Getting your macOS ready for C first.

Experiment: leave pieces out

The exercise is not only to print the greeting. Break the program on purpose and watch the compiler (or the output) complain. That is how you learn which pieces are load-bearing.

Leave out stdio.h

int main(void)
{
    printf("hello, world\n");
    return 0;
}

gcc no-stdio.c -o no-stdio

Clang will refuse: printf is declared in <stdio.h>. Without the header, the compiler does not know the function’s type.

Leave out the newline

#include <stdio.h>

int main(void)
{
    printf("hello, world");
    return 0;
}

gcc no-newline.c -o no-newline
./no-newline

The text still appears, but your shell prompt often sticks to the same line — something like hello, world%. The \n was doing real work.

Leave out main

#include <stdio.h>

printf("hello, world\n");

gcc no-main.c -o no-main

A pile of errors follows. C needs an entry point; main is where the program starts and ends.

Only printf

printf("hello, world")

gcc just-printf.c -o just-printf

Again the compiler objects: no header, no main, not even a complete statement. Languages that run line-by-line (like Python’s print('hello, world')) hide that ceremony. C does not.

What you should take away

You can write, compile, and run a tiny C program. You have also seen why #include, main, and \n are not decoration — the compiler and the terminal both notice when they are gone.

Next

→ Unknown escape sequences — Exercise 1-2